1. Field of the Invention
The present invention relates to a clock information transfer system for an AAL type 1 (ATM Adaptation Layer type 1) transmission mode, and more particularly to a clock information transfer system for an AAL type 1 transmission mode for transmitting clock information from a transmitting device over an ATM (Asynchronous Transfer Mode) transmission path when a CBR (Constant Bit Rate) signal is transmitted as an AAL type 1 ATM cell.
2. Description of the Related Art
For transmitting a CBR signal as an AAL type 1 ATM cell, clock information is transferred from a transmitting device over an ATM transmission path to a receiving device. The receiving device may regenerate a clock signal from the clock information from the transmitting device according to an SRTS (Synchronous Residual Time Stamp) method and an adaptive clock method as described in recommendations of ITU-T (formerly CCITT).
The SRTS method is used when the transmitting and receiving devices are connected to the same ATM network. According to the SRTS method, RTS (Residual Time Stamp) information representing how many clock pulses of a network clock signal correspond to a certain number of clock pulses of the CBR signal of the transmitting device is multiplexed with an ATM cell, and transmitted to the receiving device, in which the clock signal of the transmitting device is regenerated from the received RTS information and the network clock signal.
The adaptive clock method is used when the transmitting and receiving devices are not connected to the same ATM network. According to the adaptive clock method, a CBR signal is separated from an ATM cell and thereafter stored in a buffer memory, after which the stored CBR signal is read from the buffer memory such that the CBR signal stored in the buffer memory will be kept at a constant level. The clock signal with which to read the stored CBR signal from the buffer memory is the clock signal of the transmitting device.
The transfer of the clock information according to the SRTS method requires that the transmitting and receiving devices use a common network clock signal. The SRTS method cannot be used for international communications because the transmitting and receiving devices use different network clock signals.
The adaptive clock method can be used for international communications. However, because of control complexities of the adaptive clock method, it needs a more expensive facility than a CLAD (Cell Assembler and Disassembler) used by the SRTS method, and suffers a poor quality of the regenerated clock signal.
If the transmitting and receiving devices are connected to the same ATM network, then clock information may be transferred from the transmitting device to the receiving device using only the CLAD according to the SRTS method. However, if communications in the same ATM network and communications between different ATM networks are to be selectively performed, then it is necessary to prepare a communication terminal capable of selectively operating according to the SRTS method and the adaptive clock method, and to switch between these SRTS and adaptive clock method. Such a communication terminal results in an expensive system. Furthermore, the communication terminal should recognize at all times which network the remote communication terminal belongs to in switching between the SRTS and adaptive clock method. Consequently, the communication terminal needs to carry out a complicated monitoring action.